1. Field of the Invention
The present invention relates to a discharge power supply apparatus that supplies stationary discharge power following a discharge state that has been caused by the application of a trigger voltage to a discharge load. Priority is claimed on Japanese Patent Application No. 2003-273348, filed on Jul. 11, 2003, and on Japanese Patent Application No. 2003-273349, filed on Jul. 11, 2003, the contents of which are incorporated herein by reference.
2. Description of Related Art
Various types of laser apparatuses, discharge lamps, strobe light apparatuses, electric discharge machines, fusion splicers for optical fibers, thin film formation apparatuses and the like are examples of a discharge loads that use discharge energy. A variety of discharge loads are used in a wide range of fields. The discharge for this kind of load is generated in a vacuum, in a special gas such as an inert gas, or in the atmosphere. To start the discharge, it is necessary to apply a trigger voltage that is higher than the stationary discharge voltage between the discharge electrodes of the discharge load. The trigger power is small in comparison to the discharge power, but when the power supply apparatus does not have the capacity to supply an adequate trigger power, the voltage between the discharge electrodes does not rise sufficiently due to leakage between the discharge electrodes during the triggering (i.e., during the supply of the trigger voltage for starting the discharge), and thereby the discharge state is not attained. After the discharge has been generated between the discharge electrodes, the discharge is maintained for a time at a voltage that is low in comparison to the trigger voltage, and thereby a power that can cause the flow the necessary discharge current can be supplied.
A conventional discharge power supply apparatus is shown in FIG. 13. In FIG. 13, the input side rectifier circuit 51 converts a three-phase alternating voltage to direct current power by rectification, and the inverter circuit 52 converts the direct current voltage output from the input side rectifier circuit 51 to a high frequency alternating current voltage of several kHz to several 10 kHz. The inverter circuit 52 is well known, and normally is pulse width controlled (ON time ratio control). The transformer 53 inputs the high frequency alternating current voltage applied from the inverter circuit 52 at the primary winding 53a, raises the alternating current voltage by a predetermined transformation ratio, and outputs this alternating current voltage from the secondary winding 53b. The alternating current voltage of the secondary winding 53b is converted to a direct current voltage by the full-wave rectifier circuit 54 on the output side, smoothed by the capacitor 55, and applied to the discharge load 56. The discharge load 56 normally is grounded by one terminal, and a negatively biased voltage is applied to the other terminal.
In the conventional discharge power supply apparatus having such a structure, if the commercial alternating current input voltage is AC 200V, the voltage after rectification by the input side rectifier circuit 51 is approximately 260V. Therefore, if the stationary discharge voltage of the discharge load 56 is 500V, the winding ratio of the secondary winding 53b to the primary winding 53a in the transformer 53, that is, the transformation ratio n, can be approximately 2. When the trigger voltage is 1000V, the transformation ratio n must be approximately 4 in order to generate this trigger voltage.
In the conventional discharge power supply apparatus, the inverter circuit 52 is controlled at the maximum pulse width at the start of the discharge, and a trigger voltage of 1000V is generated. The discharge load 56 is triggered by this 1000V trigger voltage, and after transition to the stationary discharge state, the voltage between the discharge electrodes of the discharge load 56 falls to approximately 500V, which is the stationary discharge voltage. Thus, the ON time ratio (pulse width) of the inverter circuit 52 must be made small.
However, when the ON time ratio of the inverter circuit 52 is made small, the peak value of the output current of the inverter circuit 52 increases, and because the effective value increases, there are the problems that the power loss of the switching elements in the inverter circuit 52 becomes large, and the heat of the switching elements and the winding loss of the transformer 53 increase.
In order to eliminate these drawbacks, the apparatus shown in FIG. 14 has been proposed. In this apparatus, the essential elements identical to those in FIG. 13 are denoted by identical reference numerals, and their explanation is omitted. In this conventional apparatus, in addition to the secondary winding 53b, a second secondary winding 53c for supplying an approximately 500V trigger voltage is provided separately in the transformer 3. The voltage of this second secondary winding 53c is rectified by the trigger rectifier 57, and an approximately 500V voltage is applied to both terminals of a bypass diode 59 through the resistor 58. The 500V voltage at both terminals of the bypass diode 59 is superimposed on the 500V rectified voltage of the full-wave rectifier circuit 54, and an approximately 1000V voltage is supplied to the discharge load 56.
In this power supply apparatus, the discharge is started by the application of the trigger voltage, the bypass diode 59 becomes conductive after the transition to the stationary voltage, and the second secondary winding 53c is shorted. Thus, a resistor 58 for controlling the short-circuit current becomes necessary. During the stationary discharge, the resistor 58 consumes the wasted power, and this invites both the lowering of the efficiency and heat generation.
As can be understood from the above explanation, the conventional discharge power supply apparatus has the drawbacks that the structure and control are complicated, power loss occurs, and the cost is high.
It is an object of the present invention to provide an apparatus, in which, using a simple circuit configuration, the control method for the inverter circuit does not become complicated, a large trigger voltage can be supplied at the start of the discharge, and after the start of the stationary discharge, the apparatus can maintain the stationary discharge state while limiting as much as possible the peak of the current that flows through the inverter circuit.